Boeing B737-200QC is the only narrowbody aircraft that can be changed from full passenger configuration to full cargo configuraion in 1 hour, so it is very popular for operators of remote regions, like Aloha Airlines of Hawaii & Alaska Airlines, as these airlines have to transport cargo to the remote places without any other means of transport & having not enough passengers.
(See Boeing feature story for more details) <a href="http://www.boeing.com/commercial/news/feature/737qc.html">(http://www.boeing.com/commercial/news/feature/737qc.html)</a>
But its design is very poor in today standard, like the use of low bypass engines greatly increases noise & fuel consumption (though this offers short runway takeoff capability important for remote regions, and is not attainable by newer B737 nor A320), the very small overhead bins.
So I think a new quick change variant of single aisle aircrft like that of B737-200QC should be developed to replace the B737-200QC, with detachable overhead bins to allow use of large containers in cargo configuration, and much lower fuel consumption & noise than B737-200QC.